The mechanism of nucleation of static recrystallization in austenite after hot deformation

Abstract

Producing robust recrystallization models which can assist metallic microstructural design requires effectively understanding recrystallization nucleation. When the nucleation of static recrystallization (SRX) occurs at deformed grain boundaries, strain-induced boundary migration (bulging) is generally accepted as the nucleation mechanism. However, the present study challenges that view, showing, for a Ni-30%Fe alloy, that nucleation at deformed grain boundaries is not solely determined by bulging: results indicate that the number of bulges developed in the deformed microstructure is over four times larger than the number of SRX grains. On the other hand, SRX nucleation is shown to occur only when the low-angle boundary (LAB) between a pre-existing bulge and its parent grain transforms into a high-angle boundary (HAB). Based on this, a novel nucleation criterion is proposed, which may apply to SRX irrespective of the nucleation site (and to dynamic/metadynamic recrystallization): nucleation occurs whenever the misorientation of the LAB surrounding a bulge reaches the minimum HAB misorientation (e.g., 15°). Besides, correlation exists between the dislocation density accumulated around the various triple junction and grain boundary types in the microstructure, and their nucleation efficiency. This has been attributed to the higher fraction of relatively large initial subgrain misorientations measured for higher boundary dislocation density.